This invention relates to means for mounting a roller cutter on a drill bit for rotation, and more particularly to such means including a drill bit journal and a threaded retainer ring secured to the roller cutter received within an annular groove on the journal for retaining the roller cutter on the journal.
Heretofore, such as illustrated in U.S. Pat. No. 3,971,600 dated July 27, 1976, threaded retainer rings have been utilized for retaining roller cutters on journals of an earth boring drill bit. The retainer ring has a body of a generally rectangular or L-shaped cross section and fits within an annular groove on the journal for threading onto internal screw threads of the roller cutter for rotation with the roller cutter about the journal.
A possible source of journal failure may result from a stress concentration at the inner corner of the groove where cyclic bending stresses cause fatigue cracking that results in fracture across the entire diameter of the groove. This bending moment results from the cutter acting against the extending end of the journal thereby providing maximum bending stress at the inner corner where a stress concentration occurs.
A flange is formed on the journal outwardly of the annular groove and the thickness of the flange has to be sufficient to resist the thrust or axial loading against the flange resulting from contact with the retainer ring. Another stress concentration usually occurs at the outer corner of the groove adjacent the flange and failure of the flange beginning at this corner may occur under prolonged periods of use or under particularly high loading unless an adequate thickness flange is provided.
The stress concentration occurring at the corners may be reduced upon an increase in the size of the radii forming the corners. Thus, the design of the radii for the corners of the groove receiving a threaded retainer ring is important in providing a journal design adequate to withstand prolonged periods of use with roller cutters under adverse high loading conditions exerted against the roller cutters, particularly when certain types of especially demanding applications are encountered. A further improvement in the performance of the journal may be realized by employing surface treatments such as carburizing, nitriding, selective case hardening or shot-peening which increase local yield strength of the journal material and impart a residual stress state which offsets the applied stresses during operation.
In addition, another source of failure involves the screw threads between the roller cutter and threaded retainer ring. An adequate screw thread engagement for a predetermined screw thread class or size must be provided by the threaded retainer ring to ensure adequate thread strength.